Manufacturers

8" tube rings

R489

These split and hinged optical tube mounting rings are made of cast-aluminum, feature
quick release, hinged clamp knobs and felt lining. Designed for use
with GSO mounts, they also fit with some other brands of mounts as
well.

Inside
diameter (ID) is listed. Choose the size that match the outside
diameter of your telescope tube. If OD of the telescope is a little
smaller than ID of the rings that you can shim the inside of rings with
extra felt.